Sensing device with interchangeable nibs

ABSTRACT

A sensing device for use with a surface, the sensing device including a motion sensor configured to generate movement data indicative of movement of the sensing device relative to the surface, a nib for marking the surface, the nib having associated nib information indicative of at least one surface-marking characteristic of the nib, and a transmitter for transmitting the movement data together with the nib information to a computer system.

CO-PENDING APPLICATIONS

Various methods, systems and apparatus relating to the present inventionare disclosed in the following co-pending applications filed by theapplicant or assignee of the present invention simultaneously with thepresent invention:

--09/693,415, 09/693,219, 09/693,280, 09/693,515, 09/693,705,09/693,647, 09/693,690, 09/693,593, 09/693,216, 09/693,341, 09/696,473,09/696,514, 09/693,301, 09/693,388, 09/693,704, 09/693,510, 09/693,336,09/693,335

The disclosure of these co-pending applications are incorporated hereinby cross-reference.

Various methods, systems and apparatus relating to the present inventionare disclosed in the following co-pending applications filed by theapplicant or assignee of the present invention on Sep. 15, 2000:

Ser. Nos. 09/663,579, 09/669,599, 09/663,701, 09/663,640

The disclosures of these co-pending, applications are incorporatedherein by reference.

Various methods, systems and apparatus relating to the present inventionare disclosed in the following co-pending applications filed by theapplicant or assignee of the present invention on Jun. 30, 2000:

--09/609,139, 09/608,970, 09/609,039, 09/607,852, 09/607,656,09/609,132, 09/609,303, 09/610,095, 09/609,596, 09/607,843, 09/607,605,09/608,178, 09/609,553, 09/609,233, 09/609,149, 09/608,022, 09/609,232,09/607,844, 09/607,657, 09/608,920, 09/607,985, 09/607,990 09/607,196,09/606,999

The disclosure of these co-pending applications are incorporated hereinby cross-reference.

Various methods, systems and apparatus relating to the present inventionare disclosed in the following co-pending applications filed by theapplicant or assignee of the present invention on May 23, 2000:

--09/575,197, 09/575,195, 09/575,159, 09/575,132, 09/575,123,09/575,148, 09/575,130, 09/575,165, 09/575,153, 09/575,118, 09/575,131,09/575,116, 09/575,144, 09/575,139, 09/575,186, 09/575,185, 09/575,191,09/575,145, 09/575,192, 09/575,181, 09/575,193, 09/575,156, 09/575,183,09/575,160, 09/575,150, 09/575,169, 09/575,184, 09/575,128, 09/575,180,09/575,149, 09/575,179, 09/575,187, 09/575,155, 09/575,133, 09/575,143,09/575,196, 09/575,198, 09/575178, 09/575,164, 09/575,146, 09/575,174,09/575,163, 09/575,168, 09/575,154, 09/575,129, 09/575,124, 09/575,188,09/575,189, 09/575,162, 09/575,172, 09/575,170, 09/575,171, 09/575,161,09/575,141, 09/575,125, 09/575,142, 09/575,140, 09/575,190, 09/575,138,09/575,126, 09/575,127, 09/575,158, 09/575,117, 09/575,147, 09/575,152,09/575,176, 09/575,115, 09/575,114, 091575,113, 09/575112, 09/575,111,09/575,108, 09/575,109, 09/575,110--

The disclosure of these co-pending applications are incorporated hereinby cross-reference.

FIELD OF INVENTION

The present invention relates generally to a drawing device withinterchangeable nibs, and more particularly, to a drawing device whichsenses its own movement and combines sensed movement data with nibinformation.

BACKGROUND

Pen-like devices have been described which mark a surface whilesimultaneously capturing their own movement relative to the surface.Some such devices sense their own movement using embeddedaccelerometers. Others sense their own movement by detecting informationwhich is machine-readably encoded on the surface. In this lattercategory, see for example U.S. Pat. Nos. 5,477,012, 5,652,412 and PCTapplication WO99/50787.

While these pen-like devices may support interchangeable pen cartridgeswith varying color and line width characteristics, they do not capturethese characteristics with the movement data.

SUMMARY OF INVENTION

The present invention provides, in a first aspect, a sensing device foruse with a surface, the sensing device including a motion sensorconfigured to generate movement data indicative of movement of thesensing device relative to the surface, a nib for marking the surface,the nib having associated nib information indicative of at least onesurface-marking characteristic of the nib, and a transmitter fortransmitting the movement data together with the nib information to acomputer system.

The sensing device preferably includes a body portion and the nib is aseparate component which is attachable to and detachable from the bodyportion.

The sensing device preferably includes an interrogating device forobtaining the nib information from the nib.

The surface-marking characteristic is preferably a nib shape, a nibsize, a line width, a color, or a texture.

The nib preferably includes a storage device for storing the nibinformation.

The sensing device preferably includes a storage device for storing thenib information and the movement data.

The body portion of the sensing device is preferably in the shape of apen and the nib is attachable to a longitudinal end portion of the pen.

The sensing device preferably includes a code sensor configured togenerate, by sensing coded data disposed on the surface, location dataindicative of a location of the sensing device relative to the surface,the coded data being indicative of at least one reference point of thesurface, the motion sensor being configured to generate the movementdata using the location data.

The coded data preferably includes a plurality of tags, each tag beingindicative of a location of the tag on the surface.

The sensing device preferably includes a code sensor configured togenerate, by sensing coded data disposed on the surface, identity dataindicative of an identity of a region of the surface, the coded databeing indicative of an identity of at least one region of the surface,the motion sensor being configured to include the identity data in themovement data.

The coded data preferably includes a plurality of tags, each tag beingindicative of an identity of a region of the surface within which thetag lies.

The sensing device preferably includes at least one acceleration sensorconfigured to generate acceleration data indicative of acceleration ofthe sensing device as the sensing device moves relative to the surface,the motion sensor being configured to generate the movement data usingthe acceleration data.

The acceleration sensor is preferably configured to sense at least twosubstantially orthogonal components of acceleration.

The sensing device preferably includes an image sensor, the image sensorbeing configured to generate, by imaging the surface in the vicinity ofthe sensing device, image data, the motion sensor being configured togenerate the movement data using the image data.

The nib information preferably includes nib style information whichdescribes at least one surface-marking characteristic of the nib.

The nib information preferably includes a nib identifier, the computersystem maintains nib style information which describes at least onesurface-marking characteristic of the nib, and the nib style informationis accessible using the nib identifier.

The nib information is preferably also indicative of a functionassociated with the nib.

The present invention provides, in a second aspect, a system forcapturing a facsimile of a stroke made on a surface using a sensingdevice in accordance with the first aspect of the present invention, thesystem including the sensing device, and a computer system including areceiver configured to receive movement data and nib information fromthe sensing device, the computer system being configured to interpretthe movement data and nib information as the facsimile of the stroke.

The system preferably includes a surface having coded data disposed uponit.

Features and advantages of the present invention will become apparentfrom the following description of embodiments thereof, by way of exampleonly, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

Preferred and other embodiments of the invention will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic of a the relationship between a sample printednetpage and its online page description;

FIG. 2 is a schematic view of a interaction between a netpage pen, anetpage printer, a netpage page server, and a netpage applicationserver;

FIG. 3 is a schematic view of a high-level structure of a printednetpage and its online page description;

FIG. 4a is a plan view showing a structure of a netpage tag;

FIG. 4b is a plan view showing a relationship between a set of the tagsshown in FIG. 4a and a field of view of a netpage sensing device in theform of a netpage pen;

FIG. 5a is a plan view showing an alternative structure of a netpagetag;

FIG. 5b is a plan view showing a relationship between a set of the tagsshown in FIG. 5a and a field of view of a netpage sensing device in theform of a netpage pen;

FIG. 5c is a plan view showing an arrangement of nine of the tags shownin FIG. 5a where targets are shared between adjacent tags;

FIG. 5d is a plan view showing the interleaving and rotation of thesymbols of the four codewords of the tag shown in FIG. 5a;

FIG. 6 is a schematic view of a tag image processing and decodingalgorithm;

FIG. 7 is a perspective view of a netpage pen and its associatedtag-sensing field-of-view cone;

FIG. 8 is a perspective exploded view of the netpage pen shown in FIG.7;

FIG. 9 is a schematic block diagram of a pen controller for the netpagepen shown in FIGS. 7 and 8;

FIG. 10 is a schematic view of a pen optical path;

FIG. 11 is a flowchart of a stroke capture algorithm;

FIG. 12 is a schematic view of a raw digital ink class diagram; and

FIG. 13 is a close-up perspective view of an ink refill inserted into aslider block of the netpage pen shown in FIG. 7; and

FIG. 14 is a close-up perspective view of an ink refill and slider blockof the netpage pen shown in FIG. 7.

DETAILED DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS

Note: Memjet™ is a trademark of Silverbrook Research Pty Ltd, Australia.

In the preferred embodiment, the invention is configured to work withthe netpage networked computer system, a summary of which is given belowand a detailed description of which is given in our earlierapplications, including in particular applications U.S. Ser. Nos.09/575,129, 09/575,174, 09/575,155, 09/575,195 and 09/575,141. It willbe appreciated that not every implementation will necessarily embody allor even most of the specific details and extensions described in theseapplications in relation to the basic system. However, the system isdescribed in its most complete form to assist in understanding thecontext in which the preferred embodiments and aspects of the presentinvention operate.

In brief summary, the preferred form of the netpage system employs acomputer interface in the form of a mapped surface, that is, a physicalsurface which contains references to a map of the surface maintained ina computer system. The map references can be queried by an appropriatesensing device. Depending upon the specific implementation, the mapreferences may be encoded visibly or invisibly, and defined in such away that a local query on the mapped surface yields an unambiguous mapreference both within the map and among different maps. The computersystem can contain information about features on the mapped surface, andsuch information can be retrieved based on map references supplied by asensing device used with the mapped surface. The information thusretrieved can take the form of actions which are initiated by thecomputer system on behalf of the operator in response to the operator'sinteraction with the surface features.

In its preferred form, the netpage system relies on the production of,and human interaction with, netpages. These are pages of text, graphicsand images printed on ordinary paper or other media, but which work likeinteractive web pages. Information is encoded on each page using inkwhich is substantially invisible to the unaided human eye. The ink,however, and thereby the coded data, can be sensed by an opticallyimaging pen and transmitted to the netpage system.

In the preferred form, active buttons and hyperlinks on each page can beclicked with the pen to request information from the network or tosignal preferences to a network server. In one embodiment, text writtenby hand on a netpage is automatically recognized and converted tocomputer text in the netpage system, allowing forms to be filled in. Inother embodiments, signatures recorded on a netpage are automaticallyverified, allowing e-commerce transactions to be securely authorized.

As illustrated in FIG. 1, a printed netpage 1 can represent ainteractive form which can be filled in by the user both physically, onthe printed page, and “electronically”, via communication between thepen and the netpage system. The example shows a “Request” formcontaining name and address fields and a submit button. The netpageconsists of graphic data 2 printed using visible ink, and coded data 3printed as a collection of tags 4 using invisible ink. The correspondingpage description 5, stored on the netpage network, describes theindividual elements of the netpage. In particular it describes the typeand spatial extent (zone) of each interactive element (i.e. text fieldor button in the example), to allow the netpage system to correctlyinterpret input via the netpage. The submit button 6, for example, has azone 7 which corresponds to the spatial extent of the correspondinggraphic 8.

As illustrated in FIG. 2, the netpage pen 101, a preferred form of whichis described in our earlier application U.S. Ser. No. 09/575,174, worksin conjunction with a netpage printer 601, an Internet-connectedprinting appliance for home, office or mobile use. The pen is wirelessand communicates securely with the netpage printer via a short-rangeradio link 9.

The netpage printer 601, preferred forms of which are described in ourearlier application U.S. Ser. No. 09/575,155 and our co-filedapplication U.S. Ser. No. 09/693,514, is able to deliver, periodicallyor on demand, personalized newspapers, magazines, catalogs, brochuresand other publications, all printed at high quality as interactivenetpages. Unlike a personal computer, the netpage printer is anappliance which can be, for example, wall-mounted adjacent to an areawhere the morning news is first consumed, such as in a user's kitchen,near a breakfast table, or near the household's point of departure forthe day. It also comes in tabletop, desktop, portable and miniatureversions.

Netpages printed at their point of consumption combine the ease-of-useof paper with the timeliness and interactivity of an interactive medium.

As shown in FIG. 2, the netpage pen 101 interacts with the coded data ona printed netpage 1 and communicates, via a short-range radio link 9,the interaction to a netpage printer. The printer 601 sends theinteraction to the relevant netpage page server 10 for interpretation.In appropriate circumstances, the page server sends a correspondingmessage to application computer software running on a netpageapplication server 13. The application server may in turn send aresponse which is printed on the originating printer.

The netpage system is made considerably more convenient in the preferredembodiment by being used in conjunction with high-speedmicroelectromechanical system (MEMS) based inkjet (Memjet™) printers,for example as described in our earlier application U.S. Ser. No.09/575,141. In the preferred form of this technology, relativelyhigh-speed and high-quality printing is made more affordable toconsumers. In its preferred form, a netpage publication has the physicalcharacteristics of a traditional newsmagazine, such as a set ofletter-size glossy pages printed in full color on both sides, boundtogether for easy navigation and comfortable handling.

The netpage printer exploits the growing availability of broadbandInternet access. The netpage printer can also operate with slowerconnections, but with longer delivery times and lower image quality. Thenetpage system can also be enabled using existing consumer inkjet andlaser printers, although the system will operate more slowly and willtherefore be less acceptable from a consumer's point of view. In otherembodiments, the netpage system is hosted on a private intranet. Instill other embodiments, the netpage system is hosted on a singlecomputer or computer-enabled device, such as a printer.

Netpage publication servers 14 on the netpage network are configured todeliver print-quality publications to netpage printers. Periodicalpublications are delivered automatically to subscribing netpage printersvia pointcasting and multicasting Internet protocols. Personalizedpublications are filtered and formatted according to individual userprofiles.

A netpage printer can be configured to support any number of pens, and apen can work with any number of netpage printers. In the preferredimplementation, each netpage pen has a unique identifier. A householdmay have a collection of colored netpage pens, one assigned to eachmember of the family. This allows each user to maintain a distinctprofile with respect to a netpage publication server or applicationserver.

A netpage pen can also be registered with a netpage registration server11 and linked to one or more payment card accounts. This allowse-commerce payments to be securely authorized using the netpage pen. Thenetpage registration server compares the signature captured by thenetpage pen with a previously registered signature, allowing it toauthenticate the user's identity to an e-commerce server. Otherbiometrics can also be used to verify identity. A version of the netpagepen includes fingerprint scanning, verified in a similar way by thenetpage registration server.

Although a netpage printer may deliver periodicals such as the morningnewspaper without user intervention, it can be configured never todeliver unsolicited junk mail. In its preferred form, it only deliversperiodicals from subscribed or otherwise authorized sources. In thisrespect, the netpage printer is unlike a fax machine or e-mail accountwhich is visible to any junk mailer who knows the telephone number ore-mail address.

Each object model in the system is described using a Unified ModelingLanguage (UML) class diagram. A class diagram consists of a set ofobject classes connected by relationships, and two kinds ofrelationships are of interest here: associations and generalizations. Anassociation represents some kind of relationship between objects, i.e.between instances of classes. A generalization relates actual classes,and can be understood in the following way: if a class is thought of asthe set of all objects of that class, and class A is a generalization ofclass B, then B is simply a subset of A. Each class is drawn as arectangle labelled with the name of the class. It contains a list of theattributes of the class, separated from the name by a horizontal line,and a list of the operations of the class, separated from the attributelist by a horizontal line. In the class diagrams which follow, however,operations are never modelled. An association is drawn as a line joiningtwo classes, optionally labelled at either end with the multiplicity ofthe association. The default multiplicity is one. An asterisk (*)indicates a multiplicity of “many”, i.e. zero or more. Each associationis optionally labelled with its name, and is also optionally labelled ateither end with the role of the corresponding class. An open diamondindicates an aggregation association (“is-part-of”), and is drawn at theaggregator end of the association line. A generalization relationship(“is-a”) is drawn as a solid line joining two classes, with an arrow (inthe form of an open triangle) at the generalization end. When a classdiagram is broken up into multiple diagrams, any class which isduplicated is shown with a dashed outline in all but the main diagramwhich defines it. It is shown with attributes only where it is defined.

Netpages are the foundation on which a netpage network is built. Theyprovide a paper-based user interface to published information andinteractive services. A netpage consists of a printed page (or othersurface region) invisibly tagged with references to an onlinedescription of the page. The online page description is maintainedpersistently by a netpage page server. The page description describesthe visible layout and content of the page, including text, graphics andimages. It also describes the input elements on the page, includingbuttons, hyperlinks, and input fields. A netpage allows markings madewith a netpage pen on its surface to be simultaneously captured andprocessed by the netpage system.

Multiple netpages can share the same page description. However, to allowinput through otherwise identical pages to be distinguished, eachnetpage is assigned a unique page identifier. This page ID hassufficient precision to distinguish between a very large number ofnetpages.

Each reference to the page description is encoded in a printed tag. Thetag identifies the unique page on which it appears, and therebyindirectly identifies the page description. The tag also identifies itsown position on the page. Characteristics of the tags are described inmore detail below.

Tags are printed in infrared-absorptive ink on any substrate which isinfrared-reflective, such as ordinary paper. Near-infrared wavelengthsare invisible to the human eye but are easily sensed by a solid-stateimage sensor with an appropriate filter.

A tag is sensed by an area image sensor in the netpage pen, and the tagdata is transmitted to the netpage system via the nearest netpageprinter. The pen is wireless and communicates with the netpage printervia a short-range radio link. Tags are sufficiently small and denselyarranged that the pen can reliably image at least one tag even on asingle click on the page. It is important that the pen recognize thepage ID and position on every interaction with the page, since theinteraction is stateless. Tags are error-correctably encoded to makethem partially tolerant to surface damage.

The netpage page server maintains a unique page instance for eachprinted netpage, allowing it to maintain a distinct set of user-suppliedvalues for input fields in the page description for each printednetpage.

The relationship between the page description, the page instance, andthe printed netpage is shown in FIG. 3. The printed netpage may be partof a printed netpage document 45. The page instance is associated withboth the netpage printer which printed it and, if known, the netpageuser who requested it.

In a preferred form, each tag identifies the region in which it appears,and the location of that tag within the region. A tag may also containflags which relate to the region as a whole or to the tag. One or moreflag bits may, for example, signal a tag sensing device to providefeedback indicative of a function associated with the immediate area ofthe tag, without the sensing device having to refer to a description ofthe region. A netpage pen may, for example, illuminate an “active area”LED when in the zone of a hyperlink.

In a preferred embodiment, each tag contains an easily recognizedinvariant structure which aids initial detection, and which assists inminimizing the effect of any warp induced by the surface or by thesensing process. The tags preferably tile the entire page, and aresufficiently small and densely arranged that the pen can reliably imageat least one tag even on a single click on the page. It is importantthat the pen recognize the page ID and position on every interactionwith the page, since the interaction is stateless.

In a preferred embodiment, the region to which a tag refers coincideswith an entire page, and the region ID encoded in the tag is thereforesynonymous with the page ID of the page on which the tag appears. Inother embodiments, the region to which a tag refers can be an arbitrarysubregion of a page or other surface. For example, it can coincide withthe zone of an interactive element, in which case the region ID candirectly identify the interactive element.

Each tag contains typically contains 16 bits of tag ID, at least 90 bitsof region ID, and a number of flag bits. Assuming a maximum tag densityof 64 per square inch, a 16-bit tag ID supports a region size of up to1024 square inches. Larger regions can be mapped continuously withoutincreasing the tag ID precision simply by using abutting regions andmaps. The distinction between a region ID and a tag ID is mostly one ofconvenience. For most purposes the concatenation of the two can beconsidered as a globally unique tag ID. Conversely, it may also beconvenient to introduce structure into the tag ID, for example to definethe x and y coordinates of the tag. A 90-bit region ID allows 2⁹⁰ (˜10²⁷or a thousand trillion trillion) different regions to be uniquelyidentified. Tags may also contain type information, and a region may betagged with a mixture of tag types. For example, a region may be taggedwith one set of tags encoding x coordinates and another set, interleavedwith the first, encoding y coordinates.

In one embodiment, 120 bits of tag data are redundantly encoded using a(15, 5) Reed-Solomon code. This yields 360 encoded bits consisting of 6codewords of 15 4-bit symbols each. The (15, 5) code allows up to 5symbol errors to be corrected per codeword, i.e. it is tolerant of asymbol error rate of up to 33% per codeword. Each 4-bit symbol isrepresented in a spatially coherent way in the tag, and the symbols ofthe six codewords are interleaved spatially within the tag. This ensuresthat a burst error (an error affecting multiple spatially adjacent bits)damages a minimum number of symbols overall and a minimum number ofsymbols in any one codeword, thus maximising the likelihood that theburst error can be fully corrected.

Any suitable error-correcting code code can be used in place of a (15,5) Reed-Solomon code, for example a Reed-Solomon code with more or lessredundancy, with the same or different symbol and codeword sizes;another block code; or a different kind of code, such as a convolutionalcode (see, for example, Stephen B. Wicker, Error Control Systems forDigital Communication and Storage, Prentice-Hall 1995, the contents ofwhich a herein incorporated by cross-reference).

One embodiment of the physical representation of the tag, shown in FIG.4a and described in our earlier application U.S. Ser. No. 09/575,129,includes fixed target structures 15, 16, 17 and variable data areas 18.The fixed target structures allow a sensing device such as the netpagepen to detect the tag and infer its three-dimensional orientationrelative to the sensor. The data areas contain representations of theindividual bits of the encoded tag data. To maximise its size, each databit is represented by a radial wedge in the form of an area bounded bytwo radial lines and two concentric circular arcs. Each wedge has aminimum dimension of 8 dots at 1600 dpi and is designed so that its base(its inner arc), is at least equal to this minimum dimension. The heightof the wedge in the radial direction is always equal to the minimumdimension. Each 4-bit data symbol is represented by an array of 2×2wedges. The fifteen 4-bit data symbols of each of the six codewords areallocated to the four concentric symbol rings 18 a to 18 d ininterleaved fashion. Symbols are allocated alternately in circularprogression around the tag. The interleaving is designed to maximise theaverage spatial distance between any two symbols of the same codeword.

In order to support “single-click” interaction with a tagged region viaa sensing device, the sensing device must be able to see at least oneentire tag in its field of view no matter where in the region or at whatorientation it is positioned. The required diameter of the field of viewof the sensing device is therefore a function of the size and spacing ofthe tags. Assuming a circular tag shape, the minimum diameter of thesensor field of view 193 is obtained when the tags are tiled on aequilateral triangular grid, as shown in FIG. 4b.

The tag image processing and decoding performed by a sensing device suchas the netpage pen is shown in FIG. 6. While a captured image is beingacquired from the image sensor, the dynamic range of the image isdetermined (at 20). The center of the range is then chosen as the binarythreshold for the image 21. The image is then thresholded and segmentedinto connected pixel regions (i.e. shapes 23) (at 22). Shapes which aretoo small to represent tag target structures are discarded. The size andcentroid of each shape is also computed.

Binary shape moments 25 are then computed (at 24) for each shape, andthese provide the basis for subsequently locating target structures.Central shape moments are by their nature invariant of position, and canbe easily made invariant of scale, aspect ratio and rotation.

The ring target structure 15 is the first to be located (at 26). A ringhas the advantage of being very well behaved when perspective-distorted.Matching proceeds by aspect-normalizing and rotation-normalizing eachshape's moments. Once its second-order moments are normalized the ringis easy to recognize even if the perspective distortion was significant.The ring's original aspect and rotation 27 together provide a usefulapproximation of the perspective transform.

The axis target structure 16 is the next to be located (at 28). Matchingproceeds by applying the ring's normalizations to each shape's moments,and rotation-normalizing the resulting moments. Once its second-ordermoments are normalized the axis target is easily recognized. Note thatone third order moment is required to disambiguate the two possibleorientations of the axis. The shape is deliberately skewed to one sideto make this possible. Note also that it is only possible torotation-normalize the axis target after it has had the ring'snormalizations applied, since the perspective distortion can hide theaxis target's axis. The axis target's original rotation provides auseful approximation of the tag's rotation due to pen yaw 29.

The four perspective target structures 17 are the last to be located (at30). Good estimates of their positions are computed based on their knownspatial relationships to the ring and axis targets, the aspect androtation of the ring, and the rotation of the axis. Matching proceeds byapplying the ring's normalizations to each shape's moments. Once theirsecond-order moments are normalized the circular perspective targets areeasy to recognize, and the target closest to each estimated position istaken as a match. The original centroids of the four perspective targetsare then taken to be the perspective-distorted corners 31 of a square ofknown size in tag space, and an eight-degree-of-freedom perspectivetransform 33 is inferred (at 32) based on solving the well-understoodequations relating the four tag-space and image-space point pairs (seeHeckbert, P., Fundamentals of Texture Mapping and Image Warping, MastersThesis, Dept. of EECS, U. of California at Berkeley, Technical ReportNo. UCB/CSD 89/516, June 1989, the contents of which are hereinincorporated by cross-reference).

The inferred tag-space to image-space perspective transform is used toproject (at 36) each known data bit position in tag space into imagespace where the real-valued position is used to bilinearly interpolate(at 36) the four relevant adjacent pixels in the input image. Thepreviously computed image threshold 21 is used to threshold the resultto produce the final bit value 37.

Once all 360 data bits 37 have been obtained in this way, each of thesix 60-bit Reed-Solomon codewords is decoded (at 38) to yield 20 decodedbits 39, or 120 decoded bits in total. Note that the codeword symbolsare sampled in codeword order, so that codewords are implicitlyde-interleaved during the sampling process.

The ring target 15 is only sought in a subarea of the image whoserelationship to the image guarantees that the ring, if found, is part ofa complete tag. If a complete tag is not found and successfully decoded,then no pen position is recorded for the current frame. Given adequateprocessing power and ideally a non-minimal field of view 193, analternative strategy involves seeking another tag in the current image.

The obtained tag data indicates the identity of the region containingthe tag and the position of the tag within the region. An accurateposition 35 of the pen nib in the region, as well as the overallorientation 35 of the pen, is then inferred (at 34) from the perspectivetransform 33 observed on the tag and the known spatial relationshipbetween the pen's physical axis and the pen's optical axis.

The tag structure just described is designed to allow both regulartilings of planar surfaces and irregular tilings of non-planar surfaces.Regular tilings are not, in general, possible on non-planar surfaces. Inthe more usual case of planar surfaces where regular tilings of tags arepossible, i.e. surfaces such as sheets of paper and the like, moreefficient tag structures can be used which exploit the regular nature ofthe tiling.

An alternative tag structure more suited to a regular tiling is shown inFIG. 5a. The tag 4 is square and has four perspective targets 17. It issimilar in structure to tags described by Bennett et al. in U.S. Pat.No. 5,051,746. The tag represents sixty 4-bit Reed-Solomon symbols 47,for a total of 240 bits. The tag represents each one bit as a dot 48,and each zero bit by the absence of the corresponding dot. Theperspective targets are designed to be shared between adjacent tags, asshown in FIGS. 5b and 5 c. FIG. 5b shows a square tiling of 16 tags andthe corresponding minimum field of view 193, which must span thediagonals of two tags. FIG. 5c shows a square tiling of nine tags,containing all one bits for illustration purposes.

Using a (15, 7) Reed-Solomon code, 112 bits of tag data are redundantlyencoded to produce 240 encoded bits. The four codewords are interleavedspatially within the tag to maximize resilience to burst errors.Assuming a 16-bit tag ID as before, this allows a region ID of up to 92bits. The data-bearing dots 48 of the tag are designed to not overlaptheir neighbors, so that groups of tags cannot produce structures whichresemble targets. This also saves ink. The perspective targets thereforeallow detection of the tag, so further targets are not required. Tagimage processing proceeds as described above, with the exception thatsteps 26 and 28 are omitted.

Although the tag may contain an orientation feature to allowdisambiguation of the four possible orientations of the tag relative tothe sensor, it is also possible to embed orientation data in the tagdata. For example, the four codewords can be arranged so that each tagorientation contains one codeword placed at that orientation, as shownin FIG. 5d, where each symbol is labelled with the number of itscodeword (1-4) and the position of the symbol within the codeword (A-O).Tag decoding then consists of decoding one codeword at each orientation.Each codeword can either contain a single bit indicating whether. it isthe first codeword, or two bits indicating which codeword it is. Thelatter approach has the advantage that if, say, the data content of onlyone codeword is required, then at most two codewords need to be decodedto obtain the desired data. This may be the case if the region ID is notexpected to change within a stroke and is thus only decoded at the startof a stroke. Within a stroke only the codeword containing the tag ID isthen desired. Furthermore, since the rotation of the sensing devicechanges slowly and predictably within a stroke, only one codewordtypically needs to be decoded per frame.

It is possible to dispense with perspective targets altogether andinstead rely on the data representation being self-registering. In thiscase each bit value (or multi-bit value) is typically represented by anexplicit glyph, i.e. no bit value is represented by the absence of aglyph. This ensures that the data grid is well-populated, and thusallows the grid to be reliably identified and its perspective distortiondetected and subsequently corrected during data sampling. To allow tagboundaries to be detected, each tag data must contain a marker pattern,and these must be redundantly encoded to allow reliable detection. Theoverhead of such marker patterns is similar to the overhead of explicitperspective targets. One such scheme uses dots positioned a variouspoints relative to grid vertices to represent different glyphs and hencedifferent multi-bit values (see Anoto Technology Description, AnotoApril 2000).

Decoding a tag results in a region ID, a tag ID, and a tag-relative pentransform. Before the tag ID and the tag-relative pen location can betranslated into an absolute location within the tagged region, thelocation of the tag within the region must be known. This is given by atag map, a function which maps each tag ID in a tagged region to acorresponding location. A tag map reflects the scheme used to tile thesurface region with tags, and this can vary according to surface type.When multiple tagged regions share the same tiling scheme and the sametag numbering scheme, they can also share the same tag map. The tag mapfor a region must be retrievable via the region ID. Thus, given a regionID, a tag ID and a pen transform, the tag map can be retrieved, the tagID can be translated into an absolute tag location within the region,and the tag-relative pen location can be added to the tag location toyield an absolute pen location within the region.

The tag ID may have a structure which assists translation through thetag map. It may, for example, encoded Cartesian coordinates or polarcoordinates, depending on the surface type on which it appears. The tagID structure is dictated by and known to the tag map, and tag IDsassociated with different tag maps may therefore have differentstructures.

Two distinct surface coding schemes are of interest, both of which usethe tag structure described earlier in this section. The preferredcoding scheme uses “location-indicating” tags as already discussed. Analternative coding scheme uses “object-indicating” (or“function-indicating”) tags.

A location-indicating tag contains a tag ID which, when translatedthrough the tag map associated with the tagged region, yields a uniquetag location within the region. The tag-relative location of the pen isadded to this tag location to yield the location of the pen within theregion. This in turn is used to determine the location of the penrelative to a user interface element in the page description associatedwith the region. Not only is the user interface element itselfidentified, but a location relative to the user interface element isidentified. Location-indicating tags therefore trivially support thecapture of an absolute pen path in the zone of a particular userinterface element.

An object-indicating (or function-indicating) tag contains a tag IDwhich directly identifies a user interface element in the pagedescription associated with the region (or equivalently, a function).All the tags in the zone of the user interface element identify the userinterface element, making them all identical and thereforeindistinguishable. Object-indicating tags do not, therefore, support thecapture of an absolute pen path. They do, however, support the captureof a relative pen path. So long as the position sampling frequencyexceeds twice the encountered tag frequency, the displacement from onesampled pen position to the next within a stroke can be unambiguouslydetermined. As an alternative, the netpage pen 101 can contain a pair ormotion-sensing accelerometers, as described in our earlier applicationU.S. Ser. No. 09/575,174.

An embodiment of the present invention, in the form of a pen-likesensing device with interchangeable nibs, will now be described. Thesensing device is hereinafter simply referred to as a “pen”.

A first embodiment of the present invention will now be described withreference to FIGS. 7, 8, 9 and 10. The pen, generally designated byreference numeral 101, includes a housing 102 in the form of a plasticsmolding having walls 103 defining an interior space 104 for mounting thepen components. The pen top 105 is in operation rotatably mounted at oneend 106 of the housing 102. A semi-transparent cover 107 is secured tothe opposite end 108 of the housing 102. The cover 107 is also of moldedplastics, and is formed from semi-transparent material in order toenable the user to view the status of the LED mounted within the housing102 (see later). The cover 107 includes a main part 109 whichsubstantially surrounds the end 108 of the housing 102 and a projectingportion 110 which projects back from the main part 109 and fits within acorresponding slot 111 formed in the walls 103 of the housing 102. Aradio antenna 112 is mounted behind the projecting portion 110, withinthe housing 102. Screw threads 113 surrounding an aperture 113A on thecover 107 are arranged to receive a metal end piece 114, includingcorresponding screw threads 115. The metal end piece 114 is removable toenable ink cartridge replacement.

Also mounted within the cover 107 is a tri-color status LED 116 on aflex PCB 117. The antenna 112 is also mounted on the flex PCB 117. Thestatus LED 116 is mounted at the top of the pen 101 for good all-aroundvisibility.

The pen can operate both as a normal marking ink pen and as anon-marking stylus. An ink pen cartridge 118 with nib 119 and a stylus120 with stylus nib 121 are mounted side by side within the housing 102.Either the ink cartridge nib 119 or the stylus nib 121 can be broughtforward through open end 122 of the metal end piece 114, by rotation ofthe pen top 105. Respective slider blocks 123 and 124 are mounted to theink cartridge 118 and stylus 120, respectively. A rotatable cam barrel125 is secured to the pen top 105 in operation and arranged to rotatetherewith. The cam barrel 125 includes a cam 126 in the form of a slotwithin the walls 181 of the cam barrel. Cam followers 127 and 128projecting from slider blocks 123 and 124 fit within the cam slot 126.On rotation of the cam barrel 125, the slider blocks 123 or 124 moverelative to each other to project either the pen nib 119 or stylus nib121 out through the hole 122 in the metal end piece 114. The pen 101 hasthree states of operation. By turning the top 105 through 90° steps, thethree states are:

(1) Stylus 120 nib 121 out.

(2) Ink cartridge 118 nib 119 out.

(3) Neither ink cartridge 118 nib 119 out nor stylus 120 nib 121 out.

As shown in FIGS. 13 and 14, the pen cartridge 118 contains a compactROM chip 196. Four contact collars 195 on the pen cartridge contact fourcontact strips 197 on the slider block 123. The contact strips arestaggered so that each strip contacts exactly one of the contactcollars. The contact collars connect to the ROM, while the contactstrips connect via wires (not shown) to a pen controller 134, thusproviding a power and serial data interface between the ROM of the pencartridge and the pen controller, allowing the pen controller tointerrogate the pen cartridge for its nib ID 175. An air hole 189 in thepen cartridge allows pressure equalization as ink is extracted throughthe nib 119.

The slider block 124 similarly provides a power and serial datainterface to the pen controller 134 via contact strips (not shown).

A second flex PCB 129, is mounted on an electronics chassis 130 whichsits within the housing 102. The second flex PCB 129 mounts an infraredLED 131 for providing infrared radiation for projection onto locationtags printed in infrared ink (see later). An image sensor 132 isprovided mounted on the second flex PCB 129 for receiving reflectedradiation from the surface. The second flex PCB 129 also mounts a radiofrequency chip 133, which includes a RF transmitter and RF receiver, andthe controller chip 134 for controlling operation of the pen 101. Anoptics block 135 (formed from molded clear plastics) sits within thecover 107 and projects an infrared beam onto the surface and receivesimages onto the image sensor 132. Power supply wires 136 connect thecomponents on the second flex PCB 129 to battery contacts 137 which aremounted within the cam barrel 125. A terminal 138 connects to thebattery contacts 137 and the cam barrel 125. A three volt rechargeablebattery 139 sits within the cam barrel 125 in contact with the batterycontacts. An induction charging coil 140 is mounted about the secondflex PCB 129 to enable recharging of the battery 139 via induction. Thesecond flex PCB 129 also mounts an infrared LED 143 and infraredphotodiode 144 for detecting displacement in the cam barrel 125 wheneither the stylus 120 or the ink cartridge 118 is used for writing, inorder to enable a determination of the force being applied to thesurface by the pen nib 119 or stylus nib 121. The IR photodiode 144detects light from the IR LED 143 via reflectors (not shown) mounted onthe slider blocks 123 and 124.

Rubber grip pads 141 and 142 are provided towards the end 108 of thehousing 102 to assist gripping the pen 101, and top 105 also includes aclip 142 for clipping the pen 101 to a pocket.

The pen of this embodiment of the invention is specifically arranged todetect coded data recorded on a surface for use in sending instructionsto a computing system. Such coded data includes location tags printed onthe surface in infrared ink. Each location tag includes printed regiondata which identifies a first identity in the form of the region inwhich the tag is printed (e.g. if the tag is printed on a sheet of paperthis data will identify the sheet of paper) and also location dataidentifying a second identity in the form of the location of the tagwithin the region. The location tags also include target structureswhich enable calculation of three-dimensional orientation of the penrelative to the surface on which the tag is printed (e.g. tilt withrespect to the surface). The tag may also contain various control data.

The pen 101 is arranged to determine the position of the image sensor 32and thereby of its nib (stylus nib 121 or ink cartridge nib 119) byimaging, in the infrared spectrum, an area of the surface in thevicinity of the nib. It records the location data from the nearestlocation tag, and is arranged to calculate the distance of the imagesensor 32 and thereby of the nib 121 or 119 from the location tagutilising optics 135 and controller chip 134. The controller chip 134calculates the orientation of the pen and the nib-to-tag distance fromthe perspective distortion observed on the imaged tag.

Control data from the location tag may include control bits instructingthe pen 101 to activate its “active area” LED (this is in fact one modeof the tri-color LED 116, which becomes yellow when the pen determines,from the control data, that the area that is being imaged is an “activearea”). Thus, a region on the surface which corresponds to the activearea of a button or hyperlink may be encoded to activate this LED,giving the user of the pen visual feedback that the button or hyperlinkis active when the pen 101 passes over it. Control data may alsoinstruct the pen 101 to capture continuous pen force readings. Thus aregion on the surface which corresponds to a signature input area can beencoded to capture continuous pen 101 force.

Pen 101 action relative to the surface may comprise a series of strokes.A stroke consists of a sequence of time-stamped pen 101 positions on thesurface, initiated by pen-down event and completed by a subsequentpen-up event. Note that pen force can be interpreted relative to athreshold to indicate whether the pen is “up” or “down”, as well asbeing interpreted as a continuous value, for example when the pen iscapturing a signature. The sequence of captured strokes will be referredto hereinafter as “digital ink”. Digital ink can be used with acomputing system to form the basis for the digital exchange of drawingsand handwriting, for on-line recognition of handwriting, and for on-lineverification of signatures.

Utilizing the RF chip 133 and antenna 112 the pen 101 can transmit thedigital ink data (which is encrypted for security and packaged forefficient transmission) to the computing system.

When the pen is in range of a receiver, the digital ink data istransmitted as it is formed. When the pen 101 moves out of range,digital ink data is buffered within the pen 101 (the pen 101 circuitryincludes a buffer arranged to store digital ink data for approximately12 minutes of the pen motion on the surface) and can be transmittedlater.

The various operations of the pen will now be described in more detailin the following sections.

As discussed above, the controller chip 134 is mounted on the secondflex PCB 129 in the pen 101. FIG. 9 is a block diagram illustrating inmore detail the architecture of the controller chip 134. FIG. 9 alsoshows representations of the RF chip 133, the image sensor 132, thetri-color status LED 116, the IR illumination LED 131, the IR forcesensor LED 143, and the force sensor photodiode 144.

The pen controller chip 134 includes a controlling processor 145. Bus146 enables the exchange of data between components of the controllerchip 134. Flash memory 147 and a 512 KB DRAM 148 are also included. Ananalog-to-digital converter 149 is arranged to convert the analog signalfrom the force sensor photodiode 144 to a digital signal.

An image sensor interface 152 interfaces with the image sensor 132. Atransceiver controller 153 and base band circuit 154 are also includedto interface with the RF chip 133 which includes an RF circuit 155 andRF resonators and inductors 156 connected to the antenna 112.

The controlling processor 145 captures and decodes location data fromtags from the surface via the image sensor 132, monitors the forcesensor photodiode 144, controls the LEDs 116, 131 and 143, and handlesshort-range radio communication via the radio transceiver 153. It is amedium-performance (˜40 MHz) general-purpose RISC processor.

The processor 145, digital transceiver components (transceivercontroller 153 and baseband circuit 154), image sensor interface 152,flash memory 147 and 512 KB DRAM 148 are integrated in a singlecontroller ASIC. Analog RF components (RF circuit 155 and RF resonatorsand inductors 156) are provided in the separate RF chip.

The image sensor is CCD with an approximate resolution of 215×215 pixels(such a sensor is produced by Matsushita Electronic Corporation, and isdescribed in a paper by Itakura, K T Nobusada, N Okusenya, R Nagayoshi,and M Ozaki, “A 1 mm 50 k-Pixel IT CCD Image Sensor for Miniature CameraSystem”, IEEE Transactions on Electronic Devices, Volt 47, number 1,January 2000, which is incorporated herein by reference) with an IRfilter.

The controller ASIC 134 enters a quiescent state after a period ofinactivity when the pen 101 is not in contact with a surface. Itincorporates a dedicated circuit 150 which monitors the force sensorphotodiode 144 and wakes up the controller 134 via the power manager 151on a pen-down event.

The radio transceiver communicates in the unlicensed 900 MHz bandnormally used by cordless telephones, or alternatively in the unlicensed2.4 GHz industrial, scientific and medical (ISM) band, and usesfrequency hopping and collision detection to provide interference-freecommunication.

As discussed above, the pen 101 optics is implemented by a mouldedoptics body 135. The optics that is implemented by the optics body 135is illustrated schematically in FIG. 10. The optics comprises a firstlens 157 for focussing radiation from the infrared LED 131, a mirror158, a beam splitter 159, an objective lens 160 and a second lens 161for focusing an image onto image sensor 132. Axial rays 162 illustratethe optical path.

The optical path is designed to deliver a sharp image to the imagesensor 132 of that part 193 of the imaged surface which intersects thefield of view cone 192, within required tilt ranges. The primaryfocussing element is the objective lens 160. This is also used inreverse to project illumination from the IR illumination LED 131 ontothe surface within the field of view. Since it is impractical to placeboth the image sensor 132 and the IR LED 131 at the focus of theobjective, a beam splitter 159 is used to split the path and separaterelay lenses 157 and 161 in each path provides refocussing at the imagesensor 132 and the IR LED 131 respectively. This also allows differentapertures to be imposed on the two paths.

The edges of the image sensor 132 act as the field stop for the capturefield, and the capture path is designed so that the resulting objectspace angular field of view is as required (i.e. just under 20° for theapplication of this embodiment—see later). The illumination path isdesigned to produce the same object space field of view as the capturepath, so that the illumination fills the object space field of view withmaximum power and uniformity.

The IR LED 131 is strobed in synchrony with frame capture. The use offocussed illumination allows both a short exposure time and a smallaperture. The short exposure time prevents motion blur, thus allowingposition tag data capture during pen movement. The small aperture allowssufficient depth of field for the full range of surface depths inducedby tilt. The capture path includes an explicit aperture stop for thispurpose.

Because the image sensor 132 has a strong response throughout thevisible and near infrared part of the spectrum, it is preceded by aninfrared filter 163 in the capture path so that it captures a cleanimage of the tag data on the surface, free from interference from othergraphics on the surface which may be printed using inks which aretransparent in the near infrared.

When the stylus nib 121 or ink cartridge nib 119 of the pen 101 is incontact with a surface, the pen 101 determines its position andorientation relative to the surface at 100 Hz to allow accuratehandwriting recognition (see the article by Tappert, C, C Y Suen and TWakahara, “The State of the Art in On-Line Hand Writing Recognition”IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol 12,number 8, August 1990, the disclosure of which is incorporated herein byreference). The force sensor photodiode 144 is utilized to indicaterelative threshold whether the pen is “up” or “down”. The force may alsobe captured as a continuous value, as discussed above, to allow the fulldynamics of a signature to be verified.

The pen 101 determines the position and orientation of its nib 119, 121on the surface by imaging, in the infrared spectrum, an area of thesurface in the vicinity of the nib 119, 121. It decodes the nearest tagdata and computes the position of the nib 119, 121 relative to thelocation tag from the observed perspective distortion on the imaged tagand the known geometry of the pen optics 135. Although the positionresolution of the tag may be low, the adjusted position resolution isquite high, and easily exceeds the 200 dpi resolution required foraccurate handwriting recognition (see above reference).

Pen 101 actions relative to a surface are captured as a series ofstrokes. A stroke consists of a sequence of time-stamped pen positionson the surface, initiated by a pen-down event and completed by thesubsequent pen-up event. A stroke is also tagged with the region ID ofthe surface whenever the region ID changes, i.e. just at the start ofthe stroke under normal circumstances. As discussed above, each locationtag includes data indicative of its position on the surface and alsoregion data indicative of the region of the surface within which the taglies.

The pen also senses and decodes any markers which may be present on thesurface and in response to sensing the makers causes the electric fieldgenerator to enable marking of the surface or erasing of marks from thesurface, or to disable marking and erasing, whichever is appropriate.

FIG. 11 is a diagram illustrating location tag and stroke processing inthe pen 101. When the pen 101 is in the pen-up state, the pen controller134 continuously monitors the force sensor photodiode 144 for a pen-downcondition (step 164). While the pen is in a pen-down state, the pencontroller 134 continuously captures 165, 166 and decodes 167 tag datafrom location tags from the surface, infers the pen 101 position andorientation relative to the surface, 168 and appends the position datato the current stroke data (including the tag data and other informationsuch as force, if it is being continuously monitored). On a pen-up eventthe pen controller 134 encrypts 170 the stroke data and transmits 171the stroke data via the RF chip 133 and antenna 112, to the computingsystem. Note that the pen samples the nib force 172 in order todetermine whether the stroke has been completed 173 and also todetermine whether a new stroke is being started 174.

Assuming a reasonably fast 8 bit multiply (3 cycles), the processingalgorithm uses about 80% of the processor's time when the pen is active.

If the pen is out of range of a computing system to transmit to, then itbuffers digital ink in its internal memory. It transmits any buffereddigital ink when it is next within range of a computing system. When thepen's internal memory is full the pen ceases to capture digital ink andinstead flashes its error LED whenever the user attempts to write withthe pen 101.

FIG. 12 is a diagram illustrating the structure of the raw digital inktransmitted from the pen 101 to the computing system. Digital ink whichis buffered in the pen 101 when the pen 101 is working offline is storedin the same form as digital ink which is transmitted to the system.

When the pen 101 connects to the computing system, the controller 134notifies the system of the pen ID, nib ID 175, current absolute time176, and the last absolute time it obtained from the system prior togoing offline. This allows the system to compute any drift in the pen'sclock and timeshift any digital ink received from the pen 101accordingly. The pen 101 then synchronizes its real-time clock with theaccurate real-time clock of the system. The pen ID allows the computingsystem to identify the pen when there is more than one pen beingoperated with the computing system. Pen ID may be important in systemswhich use the pen to identify an owner of the pen, for example, andinteract with that owner in a particular directed manner. In otherembodiments this may not be required. The nib ID allows the computingsystem to identify which nib, stylus nib 121 or ink cartridge nib 119,is presently being used. The computing system can vary its operationdepending upon which nib is being used. For example, if the inkcartridge nib 119 is being used the computing system may defer producingfeedback output because immediate feedback is provided by the inkmarkings made on the surface. Where the stylus nib 121 is being used,the computing system may produce immediate feedback output.

More generally, individual pen cartridges 118 may provide particular inkcolors, thereby determining line color, and particular nib shapes andsizes, thereby determining line width. The pen controller 134interrogates the pen cartridge via the serial data interface to obtainthe nib ID 175 of the cartridge, stored in the ROM 196 of the cartridge.As described above, the controller notifies the system of the nib IDwhenever it changes. The system is thereby able to determine thecharacteristics of the nib used to produce a stroke, and is therebysubsequently able to reproduce the characteristics of the stroke itself.

Any combination of cartridges can be inserted into the pen 101. Forexample, a user may choose to insert two different-colored pencartridges rather than a pen cartridge and a stylus. Once endowed with aROM 196, the stylus 120 becomes a specific instance of a pen cartridgewhose nib ID indicates that it is non-marking.

As an alternative to having a ROM containing its nib ID, a pen cartridge118 may instead be labeled with its nib ID in a machine-readable manner.This may take the form of a barcode or even a netpage tag. The pen 101may then contain a sensor for the label, for example an optical sensor.If the label is in the form of 1 one-dimensional barcode, then it may bepossible to read the barcode using an illumination LED and photodiodefixed in relation to the pen cartridge, past which the pen cartridgemoves when selected by the user via the rotatable pen top 105. Theoptical sensor can be arranged to read the barcode as it moves past. Apair of such sensors may be used to accommodate an interchangeable pencartridge 118 and stylus 120.

As described above, the inking nib 119 and stylus nib 121 may producefunctionally different behavior in the system. More generally,particular functions or modes may be assigned to particular nib IDs. Forexample, a user may designate a particular red-colored nib forspecifying text editing commands. The system is thereby instructed toonly interpret user input labeled with the corresponding nib ID as atext editing command. The user may also designate a particular nib as apage annotation nib. The system is thereby instructed not to attempt tointerpret user input labeled with the corresponding nib ID as fieldinput, but instead to always record such input in the background fieldof the page. Similarly, the user may designate a particular nib as aselection nib, thus allowing the user to perform selection operations ona page without inadvertently entering input in a field. The user mayalso chose to restrict signature input to a particular nib, to minimizethe risk of an impostor succeeding.

Users may chose to have multiple pens at their disposal, some dedicatedto particular colors or styles, others dedicated to particularfunctions.

Our earlier application U.S. Ser. No. 09/575,174 describes in moredetail the capture of raw digital ink via the pen 101, and itssubsequent interpretation and storage as digital ink on a page server,including the handling of nib IDs associated with the raw digital inkand the association of corresponding nib styles with the stored digitalink.

At the start of a stroke the pen controller 134 records the elapsed timesince the last absolute time notified to the system. For each pen 101position 177, in the stroke the controller 134 records the x and yoffset of the pen nib 119, 121 from the current tag, the x, y and zrotation of the pen 101, and the nib force. It only records the tag ID178 (data identifying tag location) if it has changed. Since the tagfrequency is significantly smaller than the typical position samplingfrequency, the tag ID is constant for many consecutive pen 101positions, and may be constant for the entire stroke if the stroke isshort.

Since the pen 101 samples its positions and orientation at 100 Hz, pen101 positions in a stroke are implicitly clocked at 100 Hz and do notneed an explicit timestamp. If the pen 101 fails to compute a pen 101position, e.g. because it fails to decode a tag, it must still record apen 101 position to preserve the implicit clocking. It therefore recordsthe position as unknown, 179 allowing the computing system to laterinterpolate the position from adjacent samples if necessary.

Since the 32-bit time offset of a stroke has a finite range (i.e. 49.7days), the pen 101 optionally records an absolute time 176 for a stroke.This becomes the absolute time relative to which later strokes' timeoffsets are measured.

Since the region ID is constant for many consecutive strokes, the penonly records the region ID when it changes 180. This becomes the regionID implicitly associated with later pen positions.

Since a user may change the nib 119, 121 between one stroke and thenext, the pen 101 optionally records a nib ID for a stroke 175. Thisbecomes the nib ID implicitly associated with later strokes.

Each component of a stroke has an entropy-coded prefix. A 10 mm strokeof 1 second duration spans two or three tags, contains 100 positionssamples, and therefore has a size of about 5500 bits. Online continuousdigital ink capture therefore requires a maximum transmission speed of5.5 Kbps, and offline continuous digital ink capture requires about 40Kbytes of buffer memory per minute. The pen's 512 KB DRAM 48 cantherefore hold over 12 minutes of continuous digital ink. Time, regionand nib changes happen so infrequently that they have a negligibleeffect on the required transmission speed and buffer memory. Additionalcompression of pen 101 positions can reduce transmission speed andbuffer memory requirements further.

Each raw stroke is encrypted using the Triple-DES algorithm (seeSchneier, B, Applied Cryptography, Second Edition, Wiley 1996, thedisclosure of which is incorporated herein by reference) before beingtransmitted to the computing system. The pen and computing systemexchange session keys for this purpose on a regular basis. Based on aconservative estimate of 50 cycles per encrypted bit, the encryption ofa one-second 5500 bit stroke consumes 0.7% of the processor's 45 time.

In a first alternative embodiment, the coded data is indicative of anidentity which is data identifying the substrate. This enables the pen101 to identify the substrate, such as whether the substrate is aparticular type of document or whether the substrate forms part ofanother object. Of course, since with this embodiment the tags areindicative of the characteristics of the surface and not the location ofthe tags relative to the surface, separate means are required forsensing movement of the apparatus relative to the surface.

In a second alternative embodiment, the coded data is indicative of anidentity which is data indicative of the tag type. The pen 101 can thenidentify whether the tag represents an object of interest rather than aposition on the surface. For example, if the tag represents an objectand corresponds to a user interface input element (e.g. a commandbutton), then the tag can directly identify the input element.

A suitable separate movement sensing means for use with the alternativeembodiments includes a pair of orthogonal accelerometers 190 mounted ina plane normal to the pen 101 axis. The accelerometers 190 are shown inFIGS. 10 and 9 in ghost outline.

The provision of the accelerometers enables this embodiment of the pen101 to sense motion without reference to surface tags, allowing the tagsto be sampled at a lower rate.

The acceleration measured by the accelerometers in each of the X and Ydirections is integrated with respect to time to produce aninstantaneous velocity and position.

Since the starting position of the stroke is not known, only relativepositions within a stroke are calculated. Although position integrationaccumulates errors in the sensed acceleration, accelerometers typicallyhave high resolution, and the time duration of a stroke, over whicherrors accumulate is short.

Instead of providing accelerometers to sense motion relative to thesurface, alternative motion sensing means may be provided. Such meansinclude motion sensing means which includes an optical sensor whichcooperates with the surface to generate signals indicative of movementof the optical sensor relative to the surface, motion sensing meanswhich includes at least two contacts arranged to contact the surface andsense movement in two orthogonal directions, or any other suitablemotion sensing means for sensing movement relative to a surface.

The present invention has been described with reference to a preferredembodiment and number of specific alternative embodiments. However, itwill be appreciated by those skilled in the relevant fields that anumber of other embodiments, differing from those specificallydescribed, will also fall within the spirit and scope of the presentinvention. Accordingly, it will be understood that the invention is notintended to be limited to the specific embodiments described in thepresent specification, including documents incorporated bycross-reference as appropriate. The scope of the invention is onlylimited by the attached claims.

What is claimed is:
 1. A sensing device for use with a surface, thesensing device including: a motion sensor configured to generatemovement data indicative of movement of the sensing device relative tothe surface, a nib for marking the surface, the nib having associatednib information indicative of at least one surface-markingcharacteristic of the nib, the surface-marking characteristic beingselected from the group comprising: a nib shape, a nib size, a linewidth, a color, and a texture, and a transmitter for transmitting themovement data together with the nib information to a computer system. 2.A sensing device according to claim 1, wherein the sensing deviceincludes a body portion and the nib is a separate component which isattachable to and detachable from the body portion.
 3. A sensing deviceaccording to claim 1, wherein the sensing device includes aninterrogating device for obtaining the nib information from the nib. 4.A sensing device according to claim 1, wherein the nib includes astorage device for storing the nib information.
 5. A sensing deviceaccording to claim 1, wherein the sensing device includes a storagedevice for storing the nib information and the movement data.
 6. Asensing device according to claim 2, wherein the body portion is in theshape of a pen and the nib is attachable to a longitudinal end portionof the pen.
 7. A sensing device according to claim 1, further includinga code sensor configured to generate location data indicative of alocation of the sensing device relative to the surface, by sensing codeddata disposed on the surface in the vicinity of the location, the codeddata being indicative of at least one reference point of the surface,the motion sensor being configured to generate the movement data usingthe location data.
 8. A sensing device according to claim 7, wherein thecoded data includes a plurality of tags, each tag being indicative of alocation of the tag on the surface.
 9. A sensing device according toclaim 1, further including a code sensor configured to generate, bysensing coded data disposed on the surface, identity data indicative ofan identity of a region of the surface, the coded data being indicativeof an identity of at least one region of the surface, the motion sensorbeing configured to include the identity data in the movement data. 10.A sensing device according to claim 9, wherein the coded data includes aplurality of tags, each tag being indicative of an identity of a regionof the surface within which the tag lies.
 11. A sensing device accordingto claim 1, further including at least one acceleration sensorconfigured to generate acceleration data indicative of acceleration ofthe sensing device as the sensing device moves relative to the surface,the motion sensor being configured to generate the movement data usingthe acceleration data.
 12. A sensing device according to claim 11,wherein the acceleration sensor is configured to sense at least twosubstantially orthogonal components of acceleration.
 13. A sensingdevice according to claim 1, further including an image sensor, theimage sensor being configured to generate, by imaging the surface in thevicinity of the sensing device, image data, the motion sensor beingconfigured to generate the movement data using the image data.
 14. Asensing device according to claim 1, wherein the nib informationincludes nib style information which describes at least onesurface-marking characteristic of the nib.
 15. A sensing deviceaccording to claim 1, wherein the nib information includes a nibidentifier, the computer system maintains nib style information whichdescribes at least one surface-marking characteristic of the nib, andthe nib style information is accessible using the nib identifier.
 16. Asensing device according to claim 1, wherein the nib information is alsoindicative of a function associated with the nib.
 17. A system forcapturing a facsimile of a stroke made on a surface using a sensingdevice as claimed in claim 1, the system including the sensing device,and a computer system including a receiver configured to receivemovement data and nib information from the sensing device, the computersystem being configured to interpret the movement data and nibinformation as the facsimile of the stroke.
 18. A system according toclaim 17, further including a surface having coded data disposed uponthe surface according to any one of claims 8 to 11.